Demand for plastic substrates composed of polycarbonate having transparency and excellent impact resistance for use in sunglasses having glare shielding property is now growing rapidly mainly in the U.S. Plastic photochromic sunglasses which are capable of controlling glare shielding property due to variations in transmittance according to surrounding brightness and the amount of ultraviolet light by combining photochromic properties are becoming popular rapidly. These sunglasses are now manufactured by the following method.
For example, as means for forming a functional layer having glare shielding property between two plastic sheets or films, there is known a method in which a photochromic layer is formed between two polycarbonate sheets (refer to Japan Patent No. 4661017). Stated more specifically, a photochromic coating solution containing a solvent is applied to one of the plastic sheets, let pass through a drier having a specific temperature at a specific rate to be dried, and bonded to the other plastic sheet to produce a laminate. According to this method, a laminate having an adhesive layer with excellent smoothness can be produced continuously.
Further, the above laminate having a photochromic layer can be formed into a desired shape by carrying out thermal bending in which the laminate is shaped under an increased or reduced pressure (vacuum) in a heating environment, for example, at a heating temperature close to the glass transition temperature of the laminate by using a mold which has been caved into a desired shape in advance. The laminate is generally diecut into a desired shape before or after the above thermal bending.
Further, the laminate which has been thermally bent is set in the mold of an injection molding machine, and a thermoplastic resin is injection molded on the concave side of the thermally bent laminate to integrate an optical substrate composed of the thermoplastic resin with the laminate, thereby producing sunglasses.
Sunglasses provided with both polarization properties and photochromic properties were developed in order to adjust transmittance according to surrounding brightness and the amount of ultraviolet light while suppressing the glare of reflected light.
As means for manufacturing the sunglasses provided with both polarization properties and photochromic properties, there is known a method in which a photochromic layer and a polarizing layer are formed between two polycarbonate films (refer to Japan Patent No. 4586953). Stated more specifically, after a photochromic coating solution containing a solvent is applied to a polarizing film, dried and bonded to a polycarbonate film, a urethane-based adhesive is applied to the other side of the polarizing film, dried and bonded to the other polycarbonate sheet so as to produce a laminate. According to this method, a laminate having an adhesive layer with excellent smoothness can be produced continuously.
There is also known a method in which a photochromic layer and a polarizing layer are formed between a cellulose triacetate film and a polycarbonate film (refer to Japan Patent No. 5037614). Stated more specifically, Japan Patent No. 5037614 discloses the following method. A polarizing sheet having a polarizing layer between two cellulose triacetate films is prepared, and a two-liquid curable urethane resin is applied as an adhesive to one side of the polarizing sheet to bond a polycarbonate film thereto. Then, a two-liquid curable urethane resin containing a photochromic compound is applied as an adhesive to the other side of the polarizing sheet to bond a cellulose triacetate film thereto, thereby producing a laminate. According to this method, a laminate having an adhesive layer with excellent smoothness and photochromic properties can be produced continuously.
However, according to studies conducted by the inventors of the present invention, it was found that the adhesion (adhesion between plastic sheets) of each of the laminates produced by the methods disclosed by Japan Patent No. 4661017, Japan Patent No. 4586953 and Japan Patent No. 5037614 is inferior in terms of “perspiration resistance” which is required for sunglasses, that is, a state that a laminate does not change even when in contact with human perspiration. That is, it was found that in sunglasses which comprise a conventional laminate, the interface between the adhesive layer and the polycarbonate sheet or between the adhesive layer and the cellulose triacetate film may separate.
As means for improving adhesion between the photochromic layer and the optical sheet or optical film, there is proposed a method in which a photochromic polyurethane urea resin layer formed by using a photochromic compound and a polyurethane urea resin and an optical sheet or optical film are laminated through an adhesive layer (refer to JP-A 2013-33131).